In a typical home networking scenario, both high spatial coverage (e.g., coverage to various parts of the home) and high temporal coverage (e.g., coverage nearly 100% of the time) are desirable. Wireless communications often exhibit high temporal variation, which may be due to interference (in-band or out-of-band) and/or multipath characteristics. Multiple input multiple output (MIMO) techniques are sometimes used to increase capacity through multiple spatial channels and beamforming techniques, e.g., in the IEEE 802.11n standard. In a MIMO system, beamforming to point either a transmission or reception in a certain direction is accomplished in a similar manner to that done using a dish antenna. That is, a continuous surface can be approximated using a number of smaller antennas rather than a large dish antenna. Each of the smaller antennas are collocated near one another and fed with signals having predetermined phase relationships. The antenna separation is typically on the order of a quarter wavelength at the frequency of operation. For a WiFi frequency of 5.8 GHz, for example, the antennas are separated by a few inches. The various antennas exploit spatial diversity, e.g., using different transmit angles. An N×N matrix having N transmit antennas and N receive antennas is provided in MIMO systems and is well known to one of ordinary skill in the art. Assuming multipath allows for N different independent or uncorrelated paths between a transmitter and a receiver, N antennas can sustain a maximum of N spatial channels, if the channels are uncorrelated. Each such transmit antenna transmits a linear combination of N bit streams according to the N×N matrix. If the channels are partially correlated, the number of available spatial channels drops. For example, with 50% correlation, half the spatial channels are unavailable.
Beamforming is a technique used in MIMO antenna arrays for directional signal transmission or reception. Beamforming may increase the link margin and improve the coverage and range. Spatial selectivity is achieved by using either an adaptive or a fixed receive/transmit beam pattern. High spatial selectivity through beamforming is achieved with a large number of antennas located close to one another. A traded-off exists between increasing the capacity by exploiting several spatial channels and increasing the coverage by improving the link budget with beamforming.
Known MIMO communication techniques allow up to 4 transmit antennas and 4 receive antennas. Such configurations do not provide enough throughput for reliable, high-performance networking in a home environment. For example, with a 4×4 MIMO configuration having two spatial channels and beamforming, 100% spatial coverage at any location in a home may be available for a throughput of about 20-30 Mbps, but less than 90% coverage may be possible for a throughput of 50 Mbps or more. Future throughput needs are likely to be on the order of 100 Mbps or more, so more than four transmit antennas (e.g., eight or more transmit antennas) are likely to be required for the performance sought.
In some communication systems involving multiple transmit antennas, each antenna has a dedicated transmitter and digital to analog converter (DAC). In the case of zero intermediate frequency (zero-IF) or a direct conversion architecture, two DACs used (one for the in-phase and one for the quadrature components, respectively).
FIG. 1 is a block diagram of a communication apparatus in which a digital signal processor (DSP) 180 performs processing associated with MIMO spatial multiplexing. The DSP also performs beamforming on antenna data samples and provides beamformed data to transmitters 140. Three antennas 120 and three transmitters 140 are shown in a parallel. The outputs of the transmitters 140 are provided to respective bandpass filters 130. The filters provide filtered analog signals 122 to the antennas in an antenna array 110.
Such multiplicity of transmitters and DACs as in FIG. 1 results in high cost and power, which increases proportionally with the number of antenna elements in the array 110. Thus, with conventional techniques large antenna arrays (e.g., arrays having more than a few transmit antennas) may be impractical due to the cost and amount of power required.